1. Field of the Invention
The present invention relates to a high brightness electroluminescent phosphor and an electroluminescent element therewith.
2. Description of the Related Art
An electroluminescent element has a structure in that electrodes are disposed on both surfaces of a luminescent layer, in which an electroluminescent phosphor is dispersed in a dielectrics, and at least one of these electrodes is a transparent electrode. When an alternating voltage is applied between these electrodes, the element is allowed to emit. As the primary usage of the electroluminescent elements, backlight for various kinds of display devices may be cited. Furthermore, the electroluminescent element may constitute a display device by itself.
As the electroluminescent phosphor used for the electroluminescent elements, one of which host is zinc sulfide and which contains copper or manganese as an activator is generally used. A phosphor (ZnS: Cu), in which copper, as the activator, is contained in zinc sulfide, has been used as a blue or blue-green emitting electroluminescent phosphor. Furthermore, a phosphor (ZnS: Cu, Mn), in which copper and manganese, as the activator, are contained in zinc sulfide, has been used as an orange emitting electroluminescent phosphor.
The blue through blue-green emitting electroluminescent phosphor, by making use of its emission color, is applied as the backlight for displays of, for instance, cellular phones. Furthermore, by adding a red dye to the blue or blue-green emitting electroluminescent phosphor, it may be used in a white emitting electroluminescent element. Use of these light-emitting elements (electroluminescent elements) is increasing, as cellular phones and portable information terminals spread. The orange emitting electroluminescent phosphor is applied in display devices for automobile use.
The light emitting element, in which the existing zinc sulfide based electroluminescent phosphor is used, is inferior in brightness characteristics to other light emitting elements. Characteristics required for, for instance, display devices are not necessarily satisfied. From these situations, in order to improve brightness characteristics or the like of the zinc sulfide based electroluminescent phosphors, various kinds of methods have been proposed. For instance, so far, luminous efficiency and life of the zinc sulfide based electroluminescent phosphors have been tried to improve by optimizing amounts of the activator and a coactivator being added. Furthermore, Japanese Patent Application No. 2001-31963 JP-A discloses a method in that high brightness is achieved by annealing zinc sulfide based electroluminescent phosphor at temperatures in the range of substantially 150 to 300xc2x0 C.
However, according to the existing methods, it is increasingly difficult to sufficiently satisfy the brightness, which is demanded for recent light emitting elements. In general, phosphor raw material is fired under the conditions by which copper or manganese activated zinc sulfide particles grow sufficiently, and thereby the brightness of the electroluminescent phosphor may be heightened. In this case, a particle size of the zinc sulfide based electroluminescent phosphor becomes substantially from 25 to 35 xcexcm. It is increasingly difficult for the electroluminescent phosphor according to such method to heighten the brightness to a demand level, when configured as an electroluminescent element.
Meanwhile, U.S. Pat. No. 5,643,496 discloses an electroluminescent phosphor made of a copper activated zinc sulfide phosphor, of which average particle size is 23 xcexcm or less. This small size electroluminescent phosphor is obtained, without implementing an operation such as sieving, by controlling manufacturing conditions (firing conditions and so on) of the electroluminescent phosphor. The aforementioned USP document also discloses that, by making small the electroluminescent phosphor particles, an electroluminescent element therewith may be improved in brightness and life characteristics. However, even in the electroluminescent element, which is constituted of the small size electroluminescent phosphor, which is obtained by controlling such manufacturing conditions only, sufficient brightness is not necessarily obtained. Furthermore, in the small size electroluminescent phosphor, which is manufactured by controlling the manufacturing conditions only, the brightness characteristics may deteriorate in some cases.
Accordingly, an object of the present invention is to provide a zinc sulfide based electroluminescent phosphor, of which brightness characteristics are excellent and do not deteriorate during electroluminescent operation, and which has a particle size and particle size distribution, which are effective in obtaining a brighter electroluminescent element. Another object of the present invention is to provide an electroluminescent element, of which brightness is further improved due to the use of such electroluminescent phosphor.
A high brightness electroluminescent phosphor of the present invention is an electroluminescent phosphor that includes zinc sulfide based phosphor powder, of which average particle size is in the range of 10 xcexcm or more and 20 xcexcm or less, in terms of D(50) value (the D(50) value refers to the particle size obtained from the cumulative volume particle distribution curve and corresponds to the size at which the cumulative volume of particles reaches 50%), and particle size distribution includes a particle size component of 31 xcexcm or more by 15% by mass or less.
In the present electroluminescent phosphor, by removing coarser phosphor particles (coarser particle component) by means of, for instance, classification, the average particle size in terms of the D(50) value, in the range of 10 xcexcm or more and 20 xcexcm or less, is obtained, and a ratio of the particle size component of 31 xcexcm or more is reduced to 15% by mass or less. According to the phosphor powder having such average particle size and particle size distribution, when an electroluminescent element is prepared therewith, the number of phosphor particles a volume in the luminescent layer may be increased. As a result, the brightness of the electroluminescent element may be improved.
The electroluminescent element of the present invention includes the luminescent layer that includes the aforementioned high brightness electroluminescent phosphor of the present invention. As a specific configuration of such electroluminescent element, one that includes a rear electrode layer, which is integrally disposed through a reflective insulating layer along one main surface of the luminescent layer, and a transparent electrode layer, which is integrally disposed along the other main surface of the emitting layer to face thereto, may be cited.